Allow for custom ConfigureRoutes implementations

[pmjones]

This PR adds a useCustomConfigureRoutes() method to FastRoute to allow for custom ConfigureRoutes implementations.

Also in FastRoute:

• rename $configuredRoutes and $routeConfiguration to $configureRoutes for consistency
• add configureRoutes() method to allow testing for the custom implementation
• rename buildConfiguration() to processedConfiguration(), and make public so utilities (such as a URL dumper) can examine the routes
• make RouteCollector non-final so it can be extended by consumers, e.g. to auto-add route names

(Note that I modified and force-pushed this PR after initial submission; my apologies for the hassle.)